The 8x42s will be brighter, depending on your age. If you are a middle aged or older person, your eyes may only dilate to 4mm. Therefore, any exit pupil larger than that is mostly wasted. If you are a younger person, the 5.25mm exit pupil of the 8s will be a better choice. Many younger folks will have pupils that will dilate to 5 or more. The practical difference in the field regarding magnification is negligible, although 10s are a bit harder to hold as steady. so the 8x binocular
is brighter, but depending on your age, you may not notice it.